Field of the Invention
The present invention relates to an optical fiber and a manufacturing method thereof, and in particular, relates to an optical fiber having an effective refractive index profile shape in a core for reducing an excessive loss, a so-called bending loss (a macro bend loss) which occurs at the time of bending the optical fiber.
Description of Related Art
An optical fiber has been installed even on the inside of buildings, houses, or the like according to the spread of Fiber To The Home (FTTH). According to this, an optical fiber has attracted attention in which an excessive loss, so-called bending loss (a macro bend loss) which occurs at the time of imparting bending properties, is reduced.
By using an optical fiber having a low bending loss, prevention of instantaneous interruption of a signal due to the loss which occurs at the time of bending the optical fiber, a reduction in installation costs due to handling simplification, and the like have been expected.
ITU-T Recommendation G.657 is used as the standard of an optical fiber in which the bending loss is reduced compared to a standard single mode optical fiber (SSMF) while being based on ITU-T Recommendation G.652 which is the standard of the standard single mode optical fiber (SSMF).
For example, the following methods have been proposed as a method of improving (reducing) the bending loss of the standard single mode optical fiber (SSMF).
(1) Increasing Refractive Index of Core (for example, refer to Patent Document 1 (Japanese Patent No. 4268115)).
The refractive index of the core increases, and a mode field diameter (MFD) decreases compared to SSMF, and thus, containment of light with respect to the core becomes strong, and the bending loss of the optical fiber is reduced. In this case, in order to match dispersion to G.652, it is preferable that so-called depressed refractive index profile in which the refractive index of the clad near the core is reduced be adopted (for example, refer to Non-Patent Document 1 (K. Okamoto and T. Okoshi, “Computer-aided synthesis of the optimum refractive index profile for a multimode fiber,” IEEE Trans. Microwave Theory Tech., vol. MTT-25, pp. 213-221, 1976)).
A product corresponding to a bending radius up to 15 mm which is based on G.657.A1 is used as such a type of optical fiber.
(2) Disposing Low Refractive Index Portion in Position of Clad Separated from Core (for example, refer to Patent Document 2 (Japanese Unexamined Patent Application, First Publication No. 2013-88818) and Patent Document 3 (U.S. Pat. No. 8,428,411)).
A low refractive index portion, so-called trench, is disposed in a position of the clad disposed on the outer circumference of the core which is separated from the core, and thus, in a case where bending are imparted, the containment of the light with respect to the core becomes strong, and the bending loss of the optical fiber is reduced (for example, refer to Patent Document 4 (Japanese Unexamined Patent Application, First Publication No. S63-43107)).
A product corresponding to a bending radius up to 10 mm which is based on G.657.A2 or G.657.B2 and a product corresponding to a smaller bending radius up to 7.5 mm which is based on G.657.B3 are used as such a type of optical fiber. In addition, a product corresponding to a bending radius up to 7.5 mm which is based on G.657.133 and of which other optical properties are based on the standard of G.657.A series are used as such a type of optical fiber.
(3) Adding Hole to Clad (for example, refer to Patent Document 5 (Japanese Patent No. 4417286) and Patent Document 6 (Japanese Unexamined Patent Application, First Publication No. 2006-293166)).
The containment of the light with respect to the core becomes strong, and the bending loss of the optical fiber is reduced by using so-called hole assisted fiber (HAF) obtained by providing a physical hole passing through the optical fiber in a longitudinal direction in the position of the clad disposed on the outer circumference of the core and the position which is separated from the core, or by using an optical fiber (for example, ClearCurve (Registered Trademark) manufactured by Corning Incorporated) having a fine structure formed of a plurality of independent voids (for example, refer to Patent Document 7 (PCT International Publication No. WO2004/092793) and Patent Document 8 (Published Japanese Translation No. 2009-543126 of the PCT International Publication)).
A product corresponding to a bending radius up to 7.5 mm which is based on G.657.B3 is used as such a type of optical fiber.
(4) Forming Refractive Index Profile Shape of Core to α-th Power (for example, refer to Patent Document 3 and Patent Document 9 (U.S. Pat. No. 8,588,569)).
The refractive index profile shape of the core is formed to the α-th power (graded index type), and thus, the containment of the light with respect to the core becomes strong, and the bending loss is reduced. For example, in Patent Document 3, it is disclosed that in a case where the refractive index profile shape of the core is formed to the α-th power, the bending loss is reduced by 30% compared to a simple step type refractive index profile shape.
Each of the related arts has the following problems.
(1) Increasing Refractive Index of Core.
In the optical fiber having MFD smaller than that of SSMF, a connection step occurs at the time of being connected to SSMF. FIG. 1 shows a schematic view of an OTDR defective waveform observed in a case where the connection step occurs.
The OTDR defective waveform having a shape as shown in FIG. 1 is originally observed when breaking occurs (for example, refer to FIG. 5(a) of Japanese Unexamined Patent Application, First Publication No. 2000-205999), and also occurs in a position connected to an optical fiber having different MFD. This is because signal intensity of OTDR is proportionate to the minus square of MFD (inversely proportionate to the 2nd power of MFD). For example, when OTDR is measured from the side of the optical fiber having small MFD in a transmission path where an optical fiber having small MFD is connected to an optical fiber having large MFD, the waveform as shown in FIG. 1 is obtained even though breaking does not occur.
In addition, in the optical fiber in which the refractive index of the core increases, it is possible to reduce the bending loss. However, according to this, the mode field diameter decreases, and a connection loss with respect to SSMF increases. For this reason, there is a limit for reducing the bending loss.
(2) Disposing Low Refractive Index Portion in Position of Clad Separated from Core.
A Vapor-phase axial deposition (VAD) method, an Outside vapor deposition (OVD) method, a Chemical vapor deposition (CVD), and the like have been known as a preparation method of an optical fiber preform. In order to form the low refractive index portion by a method (so-called outside vapor-deposition method) of depositing a material on an outer surface of a starting member, such as a vapor-phase axial deposition method or an outside vapor deposition method, it is necessary to form a plurality of layers having a different refractive index, and thus, the number of steps necessary for manufacturing the preform increases. On the other hand, in order to form the low refractive index portion by a method (so-called inside vapor-deposition method) of depositing a material on an inner surface of a silica tube (a starting silica tube) as a starting member, such as a chemical vapor deposition method, it is necessary to form not only the core but also the trench on the inner side of the starting silica tube, and thus, the size of a preform which is able to be manufactured from the starting silica tube having the same size (of an inner diameter) becomes small. In addition, in both of the outside vapor-deposition method and the inside vapor-deposition method, a dopant for decreasing a refractive index to be less than that of silica is necessary in order to provide the low refractive index portion.
In addition, in an optical fiber having a trench type refractive index profile, it is necessary to form the plurality of layers having a different refractive index, and thus, a manufacturing step of a preform becomes complicated.
(3) Adding Hole to Clad.
A step of forming a hole in the clad is necessary in a stage of the optical fiber preform, and thus, the number of steps necessary for manufacturing the preform increases. In an optical fiber having a hole, a manufacturing step becomes complicated compared to an optical fiber having a solid structure. In addition, in order to retain the hole in a step of drawing the optical fiber from the optical fiber preform, a special drawing step is necessary. In the optical fiber having a hole, an advanced drawing technology is required, and thus, the manufacturing is not easily performed.
(4) Forming Refractive Index Profile Shape of Core to α-th Power.
In order to form the refractive index profile shape of the core to the α-th power, controllability of a refractive index profile is necessary. That is, in order to change the refractive index of a core material, it is necessary that the amount of dopant be highly controlled.